HVAC control systems are used to monitor and control temperature, humidity, air flow, air quality, and other conditions within a building or building system. Some HVAC control systems use an unoccupied setback strategy to reduce energy consumption. Unoccupied setback generally involves increasing a cooling setpoint or decreasing a heating setpoint applied to a building zone for time periods during which the building zone is unoccupied (e.g., at night, over a weekend, etc.). However, to ensure occupant comfort, the zone temperature should be within a temperature range defined by occupied cooling and heating setpoints at the time of occupancy.
The time required for a HVAC system to cool down or warm up a zone from an unoccupied setback condition is referred to as the return time. An estimate of the return time allows a HVAC control system to determine when to begin heating or cooling such that the building zone temperature is within the occupied setpoint range by the time of occupancy. It is difficult and challenging to accurately estimate the return time.